Underwater sound signal discriminating system

ABSTRACT

1. An underwater actuation system comprising means for detecting an underwater sound pressure wave and for developing an electrical signature signal correlative thereto, means for separating said signature signal into a plurality of different energy components, means for detecting the envelope of one of said energy components, means for deriving a signal corresponding to the logarithm of said detected envelope, means for deriving a signal representative of the first derivative of said logarithm signal, means for deriving a signal representative of the second derivative of said logarithm signal, means for developing a triggering signal corresponding to the product of said first and second derivative signals, means coupled to said triggering signal for developing an output, circuit means normally effective for preventing the application of said triggering signal to said output developing means, means coupled to said logarithm signal deriving means and to said one of said energy components for interrupting the passage of said one of said components within the actuation system, means for developing a control signal correlative to the ratio between the other of said energy components and the average value of said one energy component, and means coupled to said control signal for rendering said circuit means effective when a predetermined relationship between said control signal and said triggering signal is effected.

Primary @rgminer- Bichard A. Farley Attorney, Agent, or Firm-R. S.Sc1asc1a & J. A. Cooke @hwastyk Feb. 5, 1974 UNDERWATER SOUND SIGNAL fordeveloping an electrical signature signal correla- DISCRIMINATING SYSTEMtive thereto, means for separating said signature signal into aplurality of different energy components, means [75] Inventor at ChwmykSllver Spnng for detecting the envelope of one of said energycomponents, means for deriving a signal corresponding to [73] Assignee:The United States oi America as the logarithm of said detected envelope,means for derepresented by the Secretary all the riving a signalrepresentative of the first derivative of Navy, Washington, DC. saidlogarithm signal, means for deriving a signal rep- [22] Filed. Oct 151958 resentative of the second derivative of said logarithm signal,means for developing a triggering signal corre- [21] Appl. No.: 767,489sponding to the product of said first and second derivative signals,means coupled to said triggering signal 52 us. c1. 340/5 R 102/18.devebping .Output, cir?uit.means [511 at (:1......,.........:........11.11:111111 ma 11/00 e for rg 3 F of sadmggermg slgna to sm output eve oping means, means cou- 5s Fleld orSearch 340/5, 6, 102/18, 19.2 pled to Said logarithm signal derivingmeans and to [56] References Cited said one of said energy componentsfor interrupting the passage of said one of said components within theUNITED STATES PATENTS actuation system, means for developing a controlsig- 2,802,420 8/1957 MacDonald 102/18 nal correlative to the ratiobetween the other of said energy components and the average value ofsaid one energy component, and means coupled to said control signal forrendering said circuit means effective when a predetermined relationshipbetween said control signal and said triggering signal is effected.

16 Claims, 5 Drawing Figures 14 AC0 29 46 62 ,6? F u 1c BROAD BANDLOW-PASS SMOOTHING TRANSDUCER AMPLIFIER FILTER GATE DETECTOR FILTER 53 I"i DETECTOR LOGARlTHMIC CIRCUIT I men PASS FILTER SIGNAL LEvEL 14sSHIFTING CIRCUIT RATIO I 75 DETECTOR DIFFERENTIATOR I DELAY m l HI} |2sDIFFERENTIATOR GATE VOLTAGE 99 MULTIPLIER i 1 =5 MINE FIRING PATENTEU 574 SHEEY 2 BF 3 MINE FiRlNG CHANISM NdE O.

INVENTOR. ADOLPH. iVI. CHWASTYK ATTORNE Y3 UNDERWATER SOUND SIGNALDISCRIMINATING SYSTEM The invention described herein may be manufacturedand used by or for the Government of the United States of America forgovernmental purposes without the payment of any royalties thereon ortherefor.

This invention relates generally to an acoustic signal discriminatingmethod and apparatus and more particularly to a passive acoustic signaldetecting system for effecting actuation of an underwater ordnanceweapon, such for example as a naval mine, or the like.

In modern naval mine warfare widespread use is being made of electroniccircuitry for detecting the underwater acoustic signal developed byeither a surface or submerged vessel within the proximate area, and inresponse thereto, for effecting detonation of the mine if the vesselcomes within the desired firing radius of the mine. In the earlieracoustic firing systems, the mine actuating signal was almost entirelydependent on the amplitude of the acoustic signal, or signature,developed by the target ship. Because of the wide variation in soundoutput amplitude of various ships, a firing mechanism set to actuate themine by the acoustic signature of a comparatively quiet vessel withinthe desired damage range may be actuated by the acoustic.

signature of a loud vessel outside the damage range. Moreover, the sameloud vessel within the damage range may even result in countermining themine rather than actuating it.

To overcome the hereinbefore described disadvantages of amplitudeactuation, acoustic signal mine firing systems have been devised whichare selectively responsive to the mathematical derivatives of thelogarithm of the envelope of a target vesselssignature thereby resultingin mine actuation which is substantially dependent of the signalamplitude. An acoustic mine firing mechanism of this type is disclosedin the copending application of Lloyd D. Anderson, Ser. No. 487,001,filed Feb. 8, 1955, and of common assignee with this application.

Although the later developed underwater acoustic signal firing systemshave performed satisfactorily under operational conditions, recenttechnological advances in countermining and mine sweeping techniqueshave reduced the operational effectiveness of these improved acousticsystems. More specifically, methods and apparatuses for generating anacoustic countermining signal sufficiently simulative of the signatureof a target vessel have been developed for effecting a premature andnon-target destructive detonation of a naval mine. Examples of theseapparatuses are hammering mechanisms, underwater explosions, and theintermittent sound pressure wave generating device,

disclosed in the copending application of Norman Taslitt and WilliamByrd, Jr., Ser. No. 565,747, filed Feb. 15, 1956, now U.S. Pat. No.3,052,205 and ofcommon assignee with this application. To provideprotection against the premature firing, or misfiring of a naval mine bythe presence of an artificially produced underwater acoustic signal,anti-countermine circuitry have been included in the presently availableacoustic firing systems for rendering them insensitive to thecountermining signals. Although these present day anticounterminecircuits have functioned satisfactorily under most counterminingconditions, they have been found to exhibit certain inherent undesirablecharacteristics which have seriously limited their operationalusefulness. A principal limitation lies in the relatively long timeduration that the firing channel of the mine firing system is maintainedblocked, or insensitive after the expiration of the counterminingsignal. It has been found that this operational limitation will allowfor the safe passage of a target vessel by the periodic transmission ofan underwater countermining signal. Another major shortcoming of theprotective circuitry provided in the present day passive acoustic firingsystems is their inability to more readily and positively discriminatebetween an artifically generated and target generated signature. Stillanother disadvantage of present day anti-countermine circuitry lies intheir inability to provide for normal operation of an acoustic firingsystem during the concurrent presence of a target and countermine signalby reason of which a proximate target vessel employing counterminingtechniques will pass safely by the mine.

Accordingly, it is a principal object of the present invention toprovide a new and improved underwater acoustic signal responsive system.

Another object of this invention is the provision of a new and improvedpassive acoustic responsive actuating apparatus for an underwaterordnance weapon.

Still another object of the present invention resides in the provisionof an underwater acoustic signal detection circuit having improvedsignal discrimination capabilities.

A further object of the instant invention is to provide new and improvedanti-countermining circuitry for use in an underwater acoustic signalresponsive mine detonating system.

A still further object of this invention is to provide a new andimproved underwater acoustic firing system, wherein the relationshipbetween the various energy components of an underwater acousticsignature are compared and utilized for effecting discrimination betweenthe signature generated by a proximate target vessel and a counterminingdevice.

Another still further object of this invention is the provision of a newand improved underwater sound signal discrimination method.

Other objects and many of the attendant advantages of this inventionwill be more readily appreciated as the same becomes better understoodby reference to the following detailed description when considered inconnection with the accompanying drawings wherein:

FIG. la is a graphical illustration-of the energy distribution in theunderwater acoustic signature of a target vessel;

FIG. lb is a graphical illustration of the energy distribution in anartificially produced underwater acoustic signature;

FIGS. 2a and 2b show a detailed schematic diagram of the passiveacoustic firing system according to the present invention; and, FIG. 3is a block diagram of the entire system.

Referring now to the drawings,and more particularly to FIGS. la and 1bthereof, the curves 1] and 12 thereon respectively designate the highand low frequency energy distribution in the acoustic signatures of aproximate target vessel and a relatively distant countermining device ofthe types hereinbefore described. The curves show that the acousticsignatures of a vessel or countermining device consists of an energydistribution over a relatively wide range of frequencies and magnitudes.A consideration of curves 11 and 12 readily shows the existence ofvarious distinctions in the energy content of the two acoustic signals.One significant distinction between the signatures is the much largerratio of the energy in the high frequency band; i.e., above 100 cps, andthe energy in the low frequency band; i.e., between 15 and 60 cps, forthe acoustic signature generated by a target vessel. Another significantdistinction lies in the coincidence of the peaks of the high and lowfrequency energy components of the signature of a proximate vessel andthe non-coincidence of the energy peaks of the countermining signal.This phenomenon is caused by the dual paths traveled by the differentenergy components of an underwater acoustic signal. More specifically,the high frequency energy travels the direct water path while the lowfrequency energy travels through the sea bottom thereby arriving at aremote listening point ahead of the more slowly transmitted highfrequency energy. It will be apparent therefore that the more distantthe source of the acoustic energy signal from the acoustic signaldetector, the greater the time delay of arrival, A t, FIG. lb, of thehigh frequency energy peak after the low frequency energy peak.

In accordance with the general concept of the present inventiondiscrimination between a target and countermine signatures is obtainedby detection of the aforedescribed dissimilitudes between the energycontent of the divers signatures. In a generalized form of the methodand apparatus of the instant invention, a sound signal is picked up by ahydrophone and passed through an amplifier stage as required to providenecessary gain for the succeeding plurality of operations to which thedetected signal is subjected in order to obtain the desired signalsource discrimination. The detected signature is separated into itscomponential high and low frequency energy contents. The low frequencyenergy content is suitably amplified and detected. The logarithm of thelow frequency envelope is taken, the first and second derivativesthereof obtained, and a triggering signal corresponding to the productof the two derivatives developed. The ratio of the peak to the averagevalue of the low frequency energy signal is detected and thetransmission of the low frequency signal within the system regulatedaccordingly. The sequence further includes detection of the ratiobetween the high and the average of the low frequency energy signals aswell as detection of the time duration of the detected ratio thereby toprovide for selective utilization of the triggering signal by the mineinitiator. Circuitry is also provided for effecting optimum response ofthe overall firing system to the acoustic signature of a particular typetarget vessel.

FIG. 3 of the drawings discloses a block diagram of this novel system.The acoustic signal is picked up by some suitable acoustic transducersuch as shown by 13. The resulting electrical signal produced by thetransducer is fed to broad band amplifier 14. The output of thisamplifier is supplied to both low pass filter 29 and high pass filter31. Filter 31 produces a voltage containing only the high frequencycomponents of the signal picked up by transducer 13.

The output of filter 29 containing only the low frequency components ofthe signal picked up by the transducer 13 is connected to gate 46 andsignal level shifting circuit 105. Gate 46 passes the output of thefilter 29 whenever the output of detector 53 reaches or exceeds acertain predetermined value. A voltage indicative of the average valueof the output of gate 46 is obtained by detector 62, smoothing filter 67and logarithmic circuit 71 which are series connected to the gate 46.The signal produced by the logarithmic circuit is fed to signal levelshifting circuit 105 wherein a voltage commensurate with the ratio ofthe peak value to the average value of the output of filter 29 isproduced. This ratio signal is fed through amplifier 99, condenser 1 10and detector 53 to control the operation of gate 46 in the mannerpreviously mentioned. The values of resistor 126 and capacitor 113 arechosen so as to prevent this ratio signal from affecting the output ofmultiplier 111. The voltage produced by logarthmic circuit 71 issupplied to differentiator 75. One output of differentiator 75 is feddirectly to voltage multiplier 1 11 and the other output isdifferentiated a second time by circuit 95. The signal developed bydifferentiator is supplied to multiplier 111 by way of amplifier 99. Thevalue of capacitor is chosen so as to prevent passage of thisdifferentiated signal to detector 53. The first and second derivativesignals are multiplied together in circuit 1 11 and fed into the minefiring mechanism as long as the high frequency signal developed byfilter 31 is of sufficient value to maintain gate 171 open. Theoperation of differentiators 75 and 95 as well as multiplier 1 1 1produces a sharp, short duration pulse to actuate the mine firingmechanism as disclosed in the aforementioned application of Anderson.

High pass filter 31 and logarithmic circuit 71 are connected to ratiodetector which produces a voltage indicative of the ratio of the peakvalue of the high frequency portion of the signal received at transducer13 to the average value of the low frequency portion of said inputsignal. The voltage developed by detector 145 is fed into delay means167 to open gate 171 and allow passage of the signal from the multiplierto the mine firing mechanism. This takes place when the value and timeof occurrence of the peak of the high frequency portion of the receivedsignal with respect to the low frequency portion of said signal is asshown in FIG. 1a. The signatures of FIG. lb will not detonate the minebecause the high frequency peak occurs substantially after the lowfrequency peak. When an actuation pulse is produced by the multiplier inresponse to the low frequency peak, the gate remains closed because thevalue of the high frequency signal at that time is insufficient toactuate it.

The detailed circuit diagram of this invention is shown in FIGS. 2a and2b wherein the passive acoustic signal mine firing system is shown asincluding an underwater transducer, or crystal hydrophone 13 whichconverts an acoustic pressure wave impinging thereon into an electricalsignal having an instantaneous magnitude proportional to theinstantaneous intensity of the pressure wave. The electrical signatureof the impinging pressure wave is amplified to a desired level by afirst signal amplifier stage 14. The voltage amplifier 14 consists of arelatively broad band input filter network, which includes condensers15, 16 and resistors 17, 18 selected in a well known manner to pass onlythe portion of the frequency spectrum of an underwater acoustic signalrepresentative of a particular class ships signature. The amplifierstage 14 also includes an electron tube 19, the plate and screenelectrodes of which are connected to a unidirectional potential energysource B through individual dropping resistors 21 and 22, re-

spectively, and the filament of which is connected to a unidirectionalpotential energy source, such for example as a l.5 volt A battery. Acondenser 23 is included to provide a suitable by-pass to ground 24 forthe screen grid of electron tube 19. A clamping network consisting ofsemiconductor diodes 25 and 26 is connected between the positive side ofthe filament supply A and the negative side of a bias potential supplysource, such for example as a 6 volt C battery, to provide peak-to-peakamplitude limiting of the amplified output signature to a preselectedmagnitude, such for example as 7.5 volts, as will be more fullyexplained hereinafter. The amplified output signature is simultaneouslyapplied through coupling condensers 27 and 28 to a low frequency passfilter 29 and a high frequency pass filter 31 whereby the low and highfrequency energy components of the amplified acoustic signature areseparated. It is to be understood that filters 29 and 31 may constituteplug-in units, each of which is especially responsive to the energycomponents of the signature generated by a particular type targetvessel.

The low frequency band pass network 29 consists of an inductance 32,preferably exhibiting a band pass characteristic of cps i 1 octave forselectively passing only the low frequency energy content of theamplified acoustic signature. For the purpose of obtaining equal loadingon amplifier stage 14, a parallel network consisting of resistor 33 andcondenser 34 and inductance 35 may be shunt connected across the outputof the filter network 29 through coupling condenser 36.

The low frequency energy signal output of filter network 29 issimultaneously applied across two parallel voltage divider networks. Onenetwork consists of a serial arrangement of a potentiometer 37 and aresistor 38, while the other network includes a serial arrangement of ablocking condenser 39, a potentiometer 41, and resistor 42. The junctureof potentiometer 41 and resistor 42 is connected to the negativeterminal of the C potential source through resistor 43.

A portion of the low frequency energy signal, the level of which iscontrolled by the setting of potentiometer 37, is applied to the controlgrid of an electron tube 44 of a second signal amplifier stage 45 acrossa gating circuit 46. As shown, the gate 46 includes a serial arrangementof dropping resistor 47 and condensers 48,49 in the low frequency signalpath between potentiometer 37 and electron tube 44. The juncture ofcondensers 48,49 is connected through semiconductor rectifiers 51,52between the C potential supply and a detector stage 53 which develops anoutput potential corresponding to the varying ratio between the peak andaverage of the low frequency signal, as will be explained more fullyhereinafter. The low frequency signal gate 46 will permit transmissionof the low frequency signal to the control grid of tube 44 when thelevel of the output potential signal of detector 53 is above apreselected level, such for example, as the level of the negativereference potential provided by source C. Under this operatingcondition, rectifiers 51, 52 offer a high impedance path to the lowfrequency signal. However, when the output potential of detector 53 ismore negative than the reference potential, rectifier 51 becomes a lowimpedance path to the low frequency signal and most of the signal willbe dropped across resistor 47, whereupon no signal is transmitted toelec' tron tube 44. A shunting resistor 54 is provided across rectifier51 to maintain the juncture point at the negative bias potential levelthereby to minimize the amplitudes of transients developed by theopening and closing operations of gate 46.

Assuming for purposes of description that the gate 46 is open, the lowfrequency signal is impressed upon the control grid of tube 44 andamplified to a preselected level as determined by the tube bias asestablished by a voltage dividing network consisting of resistors 55,56and 57 connected across the bias potential supply. Suitable operatingpotentials are applied to the plate and screen grid of tube 44 from theB supply through individual dropping resistors 58 and 59, whilecondenser 61 provides an a.c. by-pass to ground for the screen grid.

The amplified low frequency signal output of amplifier stage 45 isapplied to a voltage doubling and rectifying stage 62 consisting ofsemiconductor diodes 63,64 and condensers 65,66. During the positivehalfcycle of the applied signal, current flows from ground 24 throughdiode 62, charging condenser 65 to approximately the peak of the inputvoltage. Diode 63 prevents current flow through condenser 66 during thishalf-cycle. During the negative half-cycle of the applied signal, thecurrent path is through condenser 64 and diode 63 thereby chargingcondenser 66 to approximately twice the peak applied signal voltage. Therectified and doubled output signal of stage 62 is applied through asmoothing filter network 67, consisting of resistor 68 and condenser 69,to a logarithmic signal developing stage 71. The logarithmic circuit 71consists of a serially connected resistor 72 and a varistor 73, theeffective resistance of which is proportional to a loga rithmic functionof the current passing through it. To obtain an extended operationalrange of logarithmic stage 71, the varistor 73 is connected to the Bsource through a dropping resistor 74. The potential output signal ofstage 71, which approximates the logarithm of the envelope of theapplied input signal to stage 71, is simultaneously applied to the highfrequency band pass filter 31 and to a differentiating network 75through a smoothing circuit consisting of resistor 76 and condenser 77.The differentiating circuit 75 includes condenser 78 and resistors 79and 80 the magnitudes of these elements being selected in a well knownmanner to have a time constant characteristic suitable for developing anoutput differentiated signal corresponding to the first derivative ofthe logarithmic signal.

The first derivative signal developed by differentiator 77 is applieddirectly to the control grid of electron tube 81 of a third signalamplifier stage 82 wherein the level of amplification is dependent uponcertain environmental conditions, such for example, as the depth atwhich the associated mine is planted. The necessary operating potentialis provided from the B supply to the screen grid of tube 81 throughresistor 83, while the plate thereof is connected to the B supplythrough a depth compensator 84. Suitable operating bias is provided forthe tube 81 by a voltage divider consisting of resistors 80,85 connectedacross the C potential source. The depth compensator includes a voltagedivider consisting of serially connected resistor s 85,86, 87 and a pairof hydrostatic pressure operated electrical switches 88,89 normallyshunted across resistors 86 and 87, respectively. Inasmuch ashydrostatic pressure switches 88 and 89 are not part of the presentinvention they are not disclosed in detail. As the mine sinks in thewater wherein it is planted, the switches are adapted to be sequentiallyoperated at varying water depths to an open position thereby resultingin the application of a larger portion of the amplified first derivativesignal. A resistance 91 shunting both switches 88 and 89 may be providedto prevent development of spurious electrical signals which may arisefrom the intermittent operation of the switches. It is to be understoodfrom the foregoing that the depth compensator provides for automaticregulation of the over-all sensitivity of the acoustic firing mechanismcorrelative with the planting depth of the mine.

The amplified first derivative logarithmic output signal of signalamplifier stage 82 is split between two paths. In one path the firstderivative signal is applied to an L filter section 92, consisting ofresistor 93 and condenser 94, the time constant of which has beenselected in a well known manner to remove any ripple voltage riding onthe first derivative signal. The first derivative signal is then appliedto a differentiating network 95. Differentiating network 95 consists ofcondenser 96 and resistors 97,98, the magnitudes of these elements beingselected in a well known manner to have a time constant characteristicsuitable for developing an output signal corresponding to the secondderivative of the first derivative logarithmic signal applied thereto.

The slowly varying second derivative logarthmic output signal ofdifferentiating network 95 is applied to a fourth signal amplificationstage 99. Amplifier stage 99 consists of electron tube 100, the plateand screen grid of which are energized from the B source throughdropping resistors 101,102, respectively. A suitable bias potential isapplied to the control grid from the negative side of the C source by avoltage divider consisting of resistors 97, 98, and 103. Condenser 104provides an a.c. by-pass to ground 24 for the screen grid of tube 100.In addition to amplifying the second derivative signal, amplifier stage99 also amplifies an a.c. blocking signal applied to the control grid oftube 100 from an automatic signal level shifting network 105 through acoupling condenser 106. Compensating network 105 includes condenser 39,potentiometer 41, resistors 42, 43, 107 and semiconductor diode 108. Thelevel of the applied a.c. blocking signal is controlled by the variationin the impedance exhibited by rectifier 108 in response to diverspotentials applied thereacross, as will now be more fully described.

A portion of the low frequency energy content of the detected targetsignature, as preselected by the setting of potentiometer 41, is appliedthrough resistor 107 to the anode of the rectifier 108, while thesmoothly varying average logarithmic signal developed across condenser77 is applied to the cathode thereof. The average value of thelogarithmic signal developed for a relatively remote target vessels willbe of a small negative value, whereupon rectifier 108 will present ahigh impedance to the low frequency signal across potentiometer 41.Under this condition substantially all of the low frequency signal,which will be of a low amplitude due to the remoteness of the targetvessel is applied to the control grid of tube 100. As the target vesseldistance lessens, the average value of the logarithmic signal becomesmore negative and the impedance presented by the semiconductor diode 108diminishes whereupon the low frequency signal will be by-passed toground 24 through rectifier 108 and condenser 77. If a high amplitudesound impulse, such as would be developed by a nearby counterminedevice, is superimposed on the low frequency signal, the rectifierbecomes a high impedance path and this portion of the impulse signal isfed to the control grid of tube 100. A resistor 109 is interposedbetween coupling condensers 96 and 106 to provide for decoupling betweenthe two signals being applied to electron tube 100. It is to beunderstood that the magnitude of the a.c. blocking signal isrepresentative of the peak to average relationship in the low frequencyenergy portion of the detected acoustic signature.

Both the slowly varying second derivative signal and the a.c. blockingsignal are amplified in amplifier stage 99 and appear at the outputthereof. The output of amplifier stage 99 is simultaneously fed to theratio detector 53 through coupling condenser 110, and to a voltagemultiplier section 111 through coupling condenser 1 12. The magnitude ofcondenser is selected in the well known manner to pass only theamplified a.c. blocking signal, while the magnitude of condenser 112 isselected to pass the amplified second derivative signal. A condenser 113is included within the multiplier stage 11 1 to by-pass to ground 24 anyof the a.c. blocking signal passing through condenser 112.

The low frequency signal ratio detector 53 is composed of a voltagepeaking and rectifying network, which includes condensers 110, 114 andsemiconductor diodes 115,116, and an R-C network, consisting of resistor117 and condenser 118. A resistor 119 is shunted across condenser 114 topermit rapid variations in the potential charge appearing thereacross.The operation of the peaking and rectifying network is analogous to thatof the voltage doubler stage 62, hereinbefore more fully described.Detector stage 53 develops a negative envelope signal having anamplitude correlative to the magnitude of the a.c. blocking signal,which signal is representative of the peak to average energyrelationship of the low frequency componential signal. The componentialelements of the ratio detector 53 are selected in a well known manner todevelop a negative output signal of a magnitude below the negativepotential level of the C source when the peak low frequency energyexceeds the average low frequency energy in the a.c. blocking signal bya certain ratio, thereby to close gate 46 in a manner as describedhereinbefore. For a ratio less than the preselected discriminating ratiolevel, negative output signal above the bias potential source will beapplied to gate 46 thereby maintaining the gate open. In view of therelatively high peak value and relatively low average value of the lowfrequency energy component of a proximate countermine signature, thediscriminating ratio level may be pre-established by potentiometer 144.The sensitivity of ratio detector stage 53 is basically dependent uponthe average logarithmic signal developed across condenser 77.Consequently as the target vessel more closely approaches and theamplitude of its associated acoustic signature increases, the logarithmof the low frequency energy component increases in a negative directionand the ratio detector becomes less sensitive to nearby counterminingsignals occurring concurrently with a close-in target vessels signature.By reason of this operational feature, the acoustic firing system cananti-countermine during the presence of a countermine signal and atarget vessels signature when the vessel is beyond the lethal range ofthe mine but will not anti-countermine when the vessel is within themines destructive radius.

The voltage multiplier circuit 111, to which the first derivative outputsignal of amplifier stage 82 and the second derivative output signal ofamplifier stage 99 is simultaneously applied, although invertedlyphased, consists of coupling condensers 122,112, between which is aserial arrangement of resistors l23,l24,125,126, semi-conductor diodes127,128 and a voltage network, which includes resistors 129, 130 andpotentiometer 13, connected between the negative potential source andground 24. A thermistor 132 may be included in the voltage divider toprovide suitable temperature compensation for the multiplier circuit 11 1. The first derivative input signal is fed through coupling condenser122 and dropping resistor 123 across rectifier 127 while the secondderivative input signal is fed through coupling condenser 112 anddropping resistor 126.

The characteristics of rectifiers 127 and 128 will provide for thedevelopment of signals thereacross corresponding to the logarithm of theapplied derivative signals. Resistors 133 and 134 may be shunted acrossrectifiers 127 and 128, respectively, for the purpose of increasing thelogarithmic response range thereof. The logarithmic signals developedacross the semiconductor diodes 127,128 are then added across resistors124,125 and the sum thereof is applied as a triggering signal to thecontrol grid of an electron tube 135 which is part of a firing relayamplifier stage 136. The setting of potentiometer 130 determines thefloating d.c. level of the multiplier stage and therefore controls themagnitude of the triggering signal developed by the multiplier circuit11 1. It is to be understood that by judicious presetting ofpotentiometers 41,47,130 and 144, certain distinctive characteristics ofthe acoustic signature of a particular class of vessels can beemphasized thereby resulting in the optimum operation of the acousticsystem in response to a particular type target at a preselected lethaldistance from the mine.

As shown in FIG. 2b of the drawings, relay amplifier 136 includes arelay 137 in the anode circuit of tube 135, which relay is operated inresponse to the application of a suitable triggering signal from themultiplier stage 111. Operation of relay 137 results in the operation ofa mine firing mechanism and a consequent detonation of the mine.Operating potentials are applied to the anode and screen of tube 135 viathe coil of relay 137 and a resistor 138, respectively.

As hereinbefore disclosed, the high frequency energy component of theamplified output of signal amplifier stage 14 is applied to a high passband filter 31 through coupling condenser 28. Filter network 31 consistsof resistors 139,140,141 and condensers 142,143, the magnitudes of whichare selected in a well known manner to pass only the high frequencyenergy portion of the detected acoustic signature, such for example assignals of a frequency of 100 cps and above. A potentiometer 144 is alsoincluded to adjust the amplitude level of the high frequency signal. Thehigh frequency signal passed by filter network 31 is compared with theaverage logarithmic low frequency signal in a ratio detector stage 145,consisting of semiconductor diode 146 and resistor 147. The ratio signaloutput of stage 145 is then applied through coupling condenser 148 tothe control grid of an electron tube 149 which comprises the activecircuit element of a fifth signal amplifier stage 150. The magnitude ofthe ratio signal developed by detector stage is largely determined bythe effective a.c. impedance presented to the high frequency signal byrectifier 146 and resistor 147. The effective a.c. impedance of the paththrough rectifier 146 and resistor 147 is dependent upon the magnitudeof the potential difference thereacross, which in turn is a function ofthe nature of the detected acoustic signature. As shown in FIG. 1a, in atarget vessels sound signature the peak of the high frequency energyoccurs concurrently with and is substantially larger, than the peak ofthe low frequency energy for the particular plug in filter networks 29and 31 employed. Under this condition, the semiconductor diode 146 willbe biased in the forward direction and will offer a relatively smallimpedance to the high frequency signal. Under this condition, asubstantial portion of the high frequency signal will be dropped acrossisolation resistor 147 and only a small portion thereof appear on thecontrol grid of electron tube 149. As shown in FIG. 1b, in the acousticsignature generated by a countermining device, particularly a distantcountermining device, the peak of the high frequency signal is delayed,or completely alternated, while due to the bottom effect, ashereinbefore described, the peak of the low frequency signal componentis extended. Under this condition, the ratio between the two signalcomponents of the artifically generated sound signature will be of aminimum value and a high impedance path will be presented by rectifier146 whereupon substantially all of the high frequency signal will beapplied to tube 149 of amplifier stage 150.

In amplifier stage 150 the necessary operating potentials for the plateand screen of tube 149 are applied thereto from the B source throughdropping resistors 151 and 182, respectively, while the control gridthereof is provided with a suitable bias potential through grid droppingresistor 153 and resistors 154,155 of a voltage dividing networkconnected across the bias potential source. Condenser 156 provides asuitable a.c. by-pass for the screen grid. The amplified high frequencyoutput signal of stage 150 is applied to a voltage doubler and detectornetwork 157, consisting of condensers 158, 159, semiconductor diodes161,162, and resistor 163 connected across the diodes. Resistor 163functions as a bleeder path for the charge accumulated across condenser159. The amplified high frequency envelope signal developed by network157 is applied to a voltage limiting network 164, consisting of aserially connected resistor 165 and a semiconductor diode 166 whichprevents the envelope signal from exceeding a preselected level, suchfor example as +0.5 volts with respect to ground 24. The high frequencyenvelope signal is then applied to an R-C timing network 167 consistingof resistor 16S and 169. The componential circuit elements of timingnetwork 167 are selected in the well known manner to have a timeconstant characteristic whereby a charge of suitable magnitude to bias asemiconductor diode gate 171 in the reverse direction is developedacross condenser 169 only upon the application of an envelope signalthereto for a preselected duration, such for example as twenty seconds.During the period when the diode gate 171 is biased in the reversedirection, it will'present a high impedance path to the triggeringsignal developed by multiplier circuit 111, and consequently, thetriggering signal will be impressed upon the control grid of tube 135thereby to actuate the mine firing mechanism. If the high frequencysignal applied to electron tube 149 is interrupted prior to theexpiration of the preselected time interval, the triggering signal willtake the low impedance path through the normally closed gate 171 tocondenser 169, which under this condition will be at a negativepotential, such for example as -4 volts. A semiconductor diode 172 isshunted across resistor 168 to provide a low impedance path for therapid discharge of condenser 169 under these conditions. The timeduration requirement obviates the possibility of mine detonation inresponse to a remote target vessel signature or a repetitive shortduration countermining signal, which may possibly duplicate the overallenergy characteristics of a proximate target signature.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. An underwater actuation system comprising means for detecting anunderwater sound pressure wave and for developing an electricalsignature signal correlative thereto, means for separating saidsignature signal into a plurality of different energy components, meansfor detecting the envelope of one of said energy components, means forderiving a signal corresponding to the logarithm of said detectedenvelope, means for deriving a signal representative of the firstderivative of said logarithm signal, means for deriving a signalrepresentative of the second derivative of said logarithm signal, meansfor developing a triggering signal corresponding to the product of saidfirst and second derivative signals, means coupled to said triggeringsignal for developing an output, circuit means normally effective forpreventing the application of said triggering signal to said outputdeveloping means, means coupled to said logarithm signal deriving meansand to said one of said energy components for interrupting the passageof said one of said components within the actuation system, means fordeveloping a control signal correlative to the ratio between the otherof said energy components and the average value of said one energycomponent, and means coupled to said control signal for rendering saidcircuit means effective when a predetermined relationship between saidcontrol signal and said triggering signal is effected.

2. An underwater mine actuation system comprising transducer means fordeveloping an audio signature signal correlative to a sound pressurewave detected thereby, means for separating said signature signal intoits low and high frequency energy components, means for selectivelydetecting the envelope of a preselected portion of the low frequencyenergy component, normally open gating means for effecting passage ofsaid low frequency energy component from said filtering means to saidlast recited means, said gating means being closed upon applicationthereto of a control signal representative of a preselected peak toaverage relationship of the energy'in said low frequency energycomponent, means for deriving a signal corresponding to the logarithm ofsaid detected envelope, means for deriving a signal representative ofthe first derivative of said logarithm signal, means for deriving asignal representative of the second derivative of said logarithm signal,means for developing a blocking signal correlative to the relationshipbetween said logarithm signal and a portion of said low frequency energycomponent,

means for comparing the peak to average energy relationship in saidblocking signal to develop said control signal correlative to saidrelationship, means for deriving a triggering signal corresponding tothe product of said first and second derivative signals, mine actuationmeans coupled to said triggering signal means for producing an outputsignal, normally closed gating means for preventing the application ofsaid triggering signal to said mine actuation means, means fordeveloping an output signal corresponding to the ratio between apreselected portion of said high frequency energy component and saidlogarithm signal, means for selectively amplifying said output signaland for detecting the envelope thereof, and means for opening saidnormally closed gating means in response to the continuous applicationof said last recited envelope for a preselected time duration.

3. A system according to claim 2 wherein said transducer means comprisesa crystal hydrophone.

4. A system according to claim 2 wherein said signature signalseparating means comprises a high band pass filter and a low band passfilter.

5. A system according to claim 2 wherein said logarithm signal derivingmeans includes a varistor.

6. A system according to claim 2 wherein said first derivative signalderiving means comprises an R-C differentiating network.

7. A system according to claim 2 wherein said second derivative signalderiving means comprises an R-C differentiating network.

8. A system according to claim 2 wherein said blocking signal developingmeans includes a variable impedance network for preselectively varyingthe portion of said low frequency energy component.

9. A system according to claim 2 wherein said blocking signal developingmeans includes a semi-conductor diode element.

10. A system according to claim 2 wherein said peak to average energyrelationship comparing means comprises a voltage doubler and detectornetwork, and an R-C network coupled thereacross having a preselectedtime constant characteristic.

11. A system according to claim 2 wherein said triggering signalderiving means includes a variable biasing network for preselectivelyestablishing the operational level thereof.

12. A system according to claim 2 wherein said normally closed gatingmeans comprises a semi-conductor diode.

13. A system according to claim 2 wherein said output signal developingmeans includes a semi-conductor diode and a variable impedance.

14. A system according to claim 2 wherein said output signal amplifyingand envelope detecting means comprises a voltage doubler and detectornetwork.

15. A system according to claim 2 wherein said opening means comprises aR-C network having a predetermined time constant characteristic.

16. An acoustic mine firing system comprising a hydrophone for detectingan underwater sound pressure wave and for developing an electricalsignature signal correlative thereto, a tuned amplifier coupled to saidhydrophone for amplifying a desired portion of said signature signal, alow pass filter network coupled to said tuned amplifier for passing thelow frequency energy portion of said signature signal, a high passfilter network coupled to said tuned amplifier for passing the highfrequency energy portion of said signature signal, a first amplifiercoupled to said low pass filter network for amplifying said lowfrequency energy portion, a first gating circuit interposed between saidhigh pass filter network and said first amplifier normally effective toallow passage of said low frequency energy portion to said firstamplifier, a first doubling and detector network coupled to said firstamplifier for developing a first unidirectional envelope signalrepresentative of the amplified low frequency energy portion, anonlinear responsive network coupled to said doubling and detectorcircuit for deriving a signal corresponding to the logarithm of saidenvelope signal, a first differentiating section coupled to saidnon-linear responsive network for deriving a first derivative signal ofsaid logarithm signal, a second differentiating section coupled to saidfirst differentiating section for deriving a second derivative signal ofsaid logarithm signal, a voltage divider network coupled to said lowpass filter network for providing a preselected magnitude of said lowfrequency energy portion, circuit means including a semiconductor diodeelement for developing a blocking signal correlative to the ratiobetween said preselected magnitude of said low frequency energy portionand the average of said logarithm signal, a signal multiplier networkcoupled to said first and second differentiating sections for deriving atriggering signal corresponding to the product of said first and secondderivative signals, mine actuation means responsive to the applicationof said triggering signal, circuit means for rendering said first gatingcircuit ineffective in response to a preselected peak to average energyrelationship in said blocking signal, circuit means including asemiconductor diode element coupled to said high pass filter network andsaid non-linear responsive network for developing a control signalcorrelative to the ratio between said high frequency energy portion andthe average of said logarithm signal, a second doubling and detectornetwork coupled to said last recited circuit means for developing asecond unidirectional envelope signal representative of said controlsignal, a timing network coupled to said second doubling and detectornetwork for developing an output signal having a magnitude correlativeto the duration and magnitude of said envelope signal, and a secondgating circuit intercoupling said mine actuation means and said timingnetwork for normally by-passing said triggering signal from applicationto said mine actuation means and in response to an output signal of apreselected magnitude for effecting application of said triggeringsignal to said mine actuation means.

1. An underwater actuation system comprising means for detecting anunderwater sound pressure wave and for developing an electricalsignature signal correlative thereto, means for separating saidsignature signal into a plurality of different energy components, meansfor detecting the envelope of one of said energy components, means forderiving a signal corresponding to the logarithm of said detectedenvelope, means for deriving a signal representative of the firstderivative of said logarithm signal, means for deriving a signalrepresentative of the second derivative of said logarithm signal, meansfor developing a triggering signal corresponding to the product of saidfirst and second derivative signals, means coupled to said triggeringsignal for developing an output, circuit means normally effective forpreventing the application of said triggering signal to said outputdeveloping means, means coupled to said logarithm signal deriving meansand to said one of said energy components for interrupting the passageof said one of said components within the actuation system, means fordeveloping a control signal correlative to the ratio between the otherof said energy components and the average value of said one energycomponent, and means coupled to said control signal for rendering saidcircuit means effective when a predetermined relationship between saidcontrol signal and said triggering signal is effected.
 2. An underwatermine actuation system comprising transducer means for developing anaudio signaturE signal correlative to a sound pressure wave detectedthereby, means for separating said signature signal into its low andhigh frequency energy components, means for selectively detecting theenvelope of a preselected portion of the low frequency energy component,normally open gating means for effecting passage of said low frequencyenergy component from said filtering means to said last recited means,said gating means being closed upon application thereto of a controlsignal representative of a preselected peak to average relationship ofthe energy in said low frequency energy component, means for deriving asignal corresponding to the logarithm of said detected envelope, meansfor deriving a signal representative of the first derivative of saidlogarithm signal, means for deriving a signal representative of thesecond derivative of said logarithm signal, means for developing ablocking signal correlative to the relationship between said logarithmsignal and a portion of said low frequency energy component, means forcomparing the peak to average energy relationship in said blockingsignal to develop said control signal correlative to said relationship,means for deriving a triggering signal corresponding to the product ofsaid first and second derivative signals, mine actuation means coupledto said triggering signal means for producing an output signal, normallyclosed gating means for preventing the application of said triggeringsignal to said mine actuation means, means for developing an outputsignal corresponding to the ratio between a preselected portion of saidhigh frequency energy component and said logarithm signal, means forselectively amplifying said output signal and for detecting the envelopethereof, and means for opening said normally closed gating means inresponse to the continuous application of said last recited envelope fora preselected time duration.
 3. A system according to claim 2 whereinsaid transducer means comprises a crystal hydrophone.
 4. A systemaccording to claim 2 wherein said signature signal separating meanscomprises a high band pass filter and a low band pass filter.
 5. Asystem according to claim 2 wherein said logarithm signal deriving meansincludes a varistor.
 6. A system according to claim 2 wherein said firstderivative signal deriving means comprises an R-C differentiatingnetwork.
 7. A system according to claim 2 wherein said second derivativesignal deriving means comprises an R-C differentiating network.
 8. Asystem according to claim 2 wherein said blocking signal developingmeans includes a variable impedance network for preselectively varyingthe portion of said low frequency energy component.
 9. A systemaccording to claim 2 wherein said blocking signal developing meansincludes a semi-conductor diode element.
 10. A system according to claim2 wherein said peak to average energy relationship comparing meanscomprises a voltage doubler and detector network, and an R-C networkcoupled thereacross having a preselected time constant characteristic.11. A system according to claim 2 wherein said triggering signalderiving means includes a variable biasing network for preselectivelyestablishing the operational level thereof.
 12. A system according toclaim 2 wherein said normally closed gating means comprises asemi-conductor diode.
 13. A system according to claim 2 wherein saidoutput signal developing means includes a semi-conductor diode and avariable impedance.
 14. A system according to claim 2 wherein saidoutput signal amplifying and envelope detecting means comprises avoltage doubler and detector network.
 15. A system according to claim 2wherein said opening means comprises a R-C network having apredetermined time constant characteristic.
 16. An acoustic mine firingsystem comprising a hydrophone for detecting an underwater soundpressure wave and for developing an electrical signature signalcorrelative thereto, a tuned amplifier coupled to said hydrophone foRamplifying a desired portion of said signature signal, a low pass filternetwork coupled to said tuned amplifier for passing the low frequencyenergy portion of said signature signal, a high pass filter networkcoupled to said tuned amplifier for passing the high frequency energyportion of said signature signal, a first amplifier coupled to said lowpass filter network for amplifying said low frequency energy portion, afirst gating circuit interposed between said high pass filter networkand said first amplifier normally effective to allow passage of said lowfrequency energy portion to said first amplifier, a first doubling anddetector network coupled to said first amplifier for developing a firstunidirectional envelope signal representative of the amplified lowfrequency energy portion, a non-linear responsive network coupled tosaid doubling and detector circuit for deriving a signal correspondingto the logarithm of said envelope signal, a first differentiatingsection coupled to said non-linear responsive network for deriving afirst derivative signal of said logarithm signal, a seconddifferentiating section coupled to said first differentiating sectionfor deriving a second derivative signal of said logarithm signal, avoltage divider network coupled to said low pass filter network forproviding a preselected magnitude of said low frequency energy portion,circuit means including a semiconductor diode element for developing ablocking signal correlative to the ratio between said preselectedmagnitude of said low frequency energy portion and the average of saidlogarithm signal, a signal multiplier network coupled to said first andsecond differentiating sections for deriving a triggering signalcorresponding to the product of said first and second derivativesignals, mine actuation means responsive to the application of saidtriggering signal, circuit means for rendering said first gating circuitineffective in response to a preselected peak to average energyrelationship in said blocking signal, circuit means including asemiconductor diode element coupled to said high pass filter network andsaid non-linear responsive network for developing a control signalcorrelative to the ratio between said high frequency energy portion andthe average of said logarithm signal, a second doubling and detectornetwork coupled to said last recited circuit means for developing asecond unidirectional envelope signal representative of said controlsignal, a timing network coupled to said second doubling and detectornetwork for developing an output signal having a magnitude correlativeto the duration and magnitude of said envelope signal, and a secondgating circuit intercoupling said mine actuation means and said timingnetwork for normally by-passing said triggering signal from applicationto said mine actuation means and in response to an output signal of apreselected magnitude for effecting application of said triggeringsignal to said mine actuation means.